Thematically, he can't drop the ball on her relationship -- he's made sure that the general audience hasn't forgotten it (and, judging by the Doctor and Rose's appearance as #1 on "50 Ways to leave your TV lover", they definitely haven't) and he's built up to her return.

I remember, when "Utopia" aired Rose's flashback scene, I was saying, "Oh, that's interesting -- RTD wants to make sure that anyone who started watching with series three knows what Rose looks like." Because showing that scene wasn't necessary -- Jack's scene with the Doctor would have still had enough weight without it. But it shows us her face again.

And anyone watching specifically for Catherine Tate would have seen Rose back in the flashback in "The Runaway Bride".